Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces having differing light reflecting characteristics. The pattern of the bars and spaces encode information. In certain bar codes, there is a single row of bars and spaces, typically of varying widths. Such bar codes are referred to as one dimensional bar codes. Other bar codes include multiple rows of bars and spaces, each typically having the same width. Such bar codes are referred to as two dimensional bar codes. Devices that read and decode one and two dimensional bar codes utilizing imaging systems that image and decode imaged bar codes are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. An illumination system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a lens of the imaging system onto the pixel array. Thus, an image of a field of view of the focusing lens is focused on the pixel array. Periodically, the pixels of the array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and attempts to decode the imaged bar code.
An emerging trend in bar code technology is direct part marking (DPM). In DPM coded indicia is imprinted directly on an item, such as a metal or plastic item. This provides permanent encoded information regarding the item that remains with the item throughout its life. In one popular method of DPM, the item is marked by subjecting a surface of the item to a series of impacts by a peening device. Each impact creates a “crater” or indentation in the item surface and a collection of craters form a pattern that conforms to a symbology of a two dimensional bar code. A commonly used two dimensional bar code symbology for DPM is the DataMatrix code. The DataMatrix bar code stores encoded information in a square or rectangular symbol that can typically range in size from 0.001 inch per side to 14 inches per side. Other methods of DPM include laser etching, chemical etching and electro-chemical etching.
DPM bar codes are more problematic to image and decode than bar codes printed on a diffuse material such as paper. This is because certain types of DPM bar codes have no intrinsic or natural contrast at the site of the markings, the markings consist of shallow indentations or craters in the background surface of the item. Further, each crater typically includes a center hole and a rim. In other types of DPM bar codes, instead of indentations, DPM markings constitute added material onto a substrate or background, where the added material is of the same kind as the substrate material. When such a DPM bar code is being read by a bar code reader or scanner, the scanner must rely on the creation of highlights and shadows on the item to properly detect the modified surface profile. If a narrow beam of light from the scanner's illumination system is used to illuminate a DPM bar code, the craters (depressions) will include both a bright highlight area and a shadow area. This is because of the shape of the crater which includes a center hole and a rim. The background, i.e., the flat or non-indented portions of the bar code, remains a relatively dark shadow area. When imaging a DPM bar code, areas of highlight and/or shadows may represent craters corresponding to a black bars or modules of a conventional 2D bar code, while the relatively unchanged background areas are assumed to correspond to spaces of a conventional bar code printed on a diffuse medium.
When reading a bar code, the bar code is illuminated by the illumination system of the bar code reader as well as being subject to ambient illumination. Typically, ambient illumination originates from multiple sources, for example, e.g., multiple ceiling fixtures having overhead fluorescent and/or incandescent lights, sunlight, etc. When bar codes are printed on diffuse material, the contrast of the features of the bar code (the bars and spaces) on the background material are not degraded by the direction or number of the illumination sources.
However, a DPM bar code is very different. When multiple illumination sources impinge upon the DPM bar code, each illumination source may produce a shadow and a highlight for each single DPM feature (each crater). When imaged by the bar code reader, such multiple shadows/highlights for each crater can complicate the decoding of the captured image of the DPM bar code.
What is desired is an imaging-based bar code reader that reduces the detrimental effect of ambient light when imaging and decoding a target image such as a target bar code.